Do you have deficiency symptoms? I think the best way is to take samples of the tissues and determined the ion concentrations by HPLC or other method like this (mass spectroscopy, etc). 

I guess it is too difficult to determine the Mg/Ca conc. in tissue without MASS or some "expensive" machines. Some molecule marker or indicater have been sold from many company, but they are no use for plant, because almost these molecules are arranged for animal cells.

Dear Mariela Ines Monteoliva, Thanks for the reply. Do you mean ICP-MS. It is a bit expensive. Can we do some crude and easy assay like some colorometric or absorbance based assays?